A conventional tandem vacuum servo unit is disclosed in Japanese Patent Laid-Open Publication No. 6-009961. This vacuum servo unit includes a housing in which is defined a pressure chamber, and a partition wall member provided in the housing to divide the pressure chamber into a front pressure chamber and a rear pressure chamber, the partition wall member being fixed in an air tight manner to the housing at the outer peripheral portion of the wall member. A movable power piston slides in an air tight manner on the inner peripheral portion of the partition wall member at an outer peripheral portion thereof.
A front movable wall member is disposed concentrically with the partition wall member in the front pressure chamber to divide the front pressure chamber into a front constant pressure chamber and a front variable pressure chamber, and a rear movable wall member is disposed concentrically with the partition wall member in the rear pressure chamber to divide the rear pressure chamber into a rear constant pressure chamber and a rear variable pressure chamber. An input member is provided in the movable power piston to move axially upon brake actuation, and an output member outputs a propulsion force generated by the movable power piston accompanying movement of the front and rear movable wall members.
A control member selectively communicates the front and rear variable pressure chambers with the negative pressure source or with atmosphere according to movement of the input member against the movable power piston. The movable power piston has a supporting portion on the outer peripheral portion thereof. An input side face of the supporting portion is perpendicular to the axis of the movable power piston.
The front constant pressure chamber is located in front of the front movable wall member and is communicated with a negative pressure source while the front variable pressure chamber is located behind the front movable wall member and is selectively communicated with atmosphere and the negative pressure source. The rear constant pressure chamber is located in front of the rear movable wall member and is communicated with the negative pressure source while the rear variable pressure chamber is located behind the rear movable wall member and is selectively communicated with atmosphere and the negative pressure source. A first engaging portion is disposed with an inner peripheral portion of the front movable wall member for engaging the supporting portion. The first engaging portion has a connecting portion engaged with the input side face of the supporting portion at an output side face thereof. The connecting portion is shaped to be straight so as to be parallel to the input side face of the supporting portion. A second engaging portion is disposed with an inner peripheral portion of the rear movable wall member for engaging an input side face of the first engaging portion at an output side face thereof. The second engaging portion is shaped to be straight so as to be parallel to the connecting portion.
In this vacuum servo unit, when the input member is moved by brake actuation, the control member cuts off communication between the front and rear variable pressure chambers and the negative pressure source, and communicates the front and rear variable pressure chambers with the atmosphere. Therefore, a pressure differential is generated between the variable pressure chambers and the constant pressure chambers. This pressure differential acts on the movable wall members, the movable power piston, and the output member.
In the vacuum servo unit described above, the first engaging portion is generally formed by pressing. If the first engaging portion is press formed with little manufacturing precision, it is possible that the connecting portion will not be parallel to the input side face of the supporting portion, i.e., the contact between the supporting portion and the connecting portion does not occur between the input side face of the supporting member and the output side face of the first engaging portion, but rather occurs between the input side face of the supporting portion and an edge of the connecting portion. Therefore, when the pressure differential is generated between the front constant pressure chamber and the front variable pressure chamber to move the movable power piston towards the output side through the front movable wall member, a large load is generated at the first engaging portion, i.e., the front movable wall member, which decreases the durability of the front movable wall member.